The invention relates to a method of and apparatus for analysing a sample, and particularly to those comprising the use of conveying means to convey a sample to an analyser.
For many years it has been known to provide a chromatography column for separating liquid mixtures containing various components dissolved or dispersed therein. During the chromatographic process, the individual components are separated by a known process using apparatus comprising a mobile phase supply, a sampling device, a column and a detector, such as HPLC, gel permeation chromatography, or ion exchange chromatography enabling the quantity and nature of each component to be determined. Hitherto, different methods of detection have been proposed.
In one method, UV detection has been found to be highly sensitive, accurate and simple to use. However, one major limitation is that for UV detection to be effective, the compound to be analysed must absorb UV. This means that the technique cannot be used for several major classes of compounds, including carbohydrates, alcohols, detergents and aliphatic hydrocarbons. Also solvent choice is limited to U.V. transparent solvents.
In another method, refractive index detection is used. Whilst it is a widely used method, it is less sensitive than the UV method described above and tends to be subject to considerable instability arising from eluent changes. It cannot be used with what is known in the art as Gradient Elution techniques.
In a further method, the components separated by chromatography are transported on a wire to a furnace where they are heated and pyrolysed. The pyrolysis products then pass by means of a complex set of conduits into an argon detector and are detected. In another embodiment the solute on the wire is burnt to carbon dioxide which is sucked into a hydrogen stream that passes over a nickel catalyst. The nickel catalyst converts the carbon dioxide quantitatively to methane which is detected by a flame ionisation detector (FID). The disadvantage of these processes is the need for a complex set of conduits and the dilution of the pyrolysis or oxidation product during the process. As the argon detector is concentration sensitive the dilution of pyrolysis produced in the carrier argon stream reduces the sensitivity. As the FID is mass sensitive and not concentration sensitive dilution is not important but the dispersion of the peak in the conduit (peak broadening) and the high noise generated by the catalytic conversion significantly reduces the sensitivity.
An object of the present invention is to provide improved methods and apparatus for detecting components of a mixture which may have been separated by chromatography.
According to a first aspect of the invention there is provided an analytical apparatus comprising:
(i) supply means for supplying a sample to be analysed;
(ii) analysing means for analysing at least one property of the sample to be analysed; and
(iii) conveying means operable to convey the sample between the supply means and the analysing means, wherein the conveying means comprises an oxidised surface layer which receives the sample.
The property may be a physical property, such as viscosity or melting temperature, of the sample or a chemical property, such as the chemical composition of the sample.
The sample may be components of a mixture which has been previously separated by chromatography.
The anlaysing means may be an analysing chamber, preferably without the need for conversion steps known hitherto.
Preferably the oxidised surface layer is in the form of a continuous surface film. The oxidised surface layer may be porous. The oxidised surface layer is preferably readily wettable by solvents, especially aqueous and non-aqueous solvents such as water and polar solvents. Examples of polar solvents include alcohols, such as ethanol and propanol, and DMSO. Preferably the oxidised surface layer is readily wettable by hydrocarbons including aromatic and non-aromatic, substituted and non-substituted alkyls.
Preferably the sample is applied to the conveying means without substantial diffusion along the conveying means.
The conveying means may comprise a metal, the metal having an oxidised surface layer.
Preferably, the conveying means is a wire which has an oxidised surface layer.
Alternatively a tape of material or a disk of material may be used instead of the wire. The latter two forms of conveyor have an oxidised surface layer, and are less likely to break than wire. Tape is especially preferred. A typical width of tape is 1.5 mm, with a thickness of 0.1 mm.
In that respect, and according to a second aspect of the invention, there is provided a method of detecting components of a mixture which have been separated by chromatography comprising the steps of providing a conveyor, such as a metal wire or tape, which has an oxidised surface layer, depositing sequentially on the oxidised surface layer the separated components of the mixture, and moving the wire and each component thereon to detection apparatus where analysis of each sample takes place.
Preferably, the conveying means, such as wire or tape, is made of titanium and the layer is oxidised titanium. The oxidised titanium forms a continuous porous surface film on the titanium metal and greatly enhances the carrying capabilities of the wire or tape. The oxidised wire wets very easily and will carry without difficulty both aqueous and non-aqueous liquids. Preoxidised titanium wire comprising an oxidised surface coating is commercially available, for example from Alloy Wire International, Cradley Heath, West Midlands, United Kingdom.
Other suitable metals comprising an oxidised surface layer include molybdenum, aluminium, iron, palladium and platinum.
The conveying means such as wire or tape is preferably carried on a supply spool and the invention may include drawing off the wire from the supply spool by means of a take up spool. A motor, such as a stepper motor, may be computer controlled to draw the wire or tape at a desired rate from the supply spool. The invention preferably includes controlling the tension of the wire between the spools to maintain a substantially constant tension in the wire particularly when the wire is heated as set out below.
A traverse mechanism may be provided to ensure that, for example, the tape/wire is regularly wound across the take up spool.
Alternatively the conveying means may be moved by means of one or more rollers in contact with the conveying means. This has the advantage that the speed of the conveying means does not vary according to the amount of e.g. tape/wire on the take up spool.
Preferably, the apparatus of the invention comprises an analyser chamber.
In a preferred embodiment the inventors have found that where a conveying means, such as tape or wire, is used according to the first or second aspect of the invention, wetting of the tape or wire with the separated components of the mixture can be improved by providing a polar cone opposite an aperture supplying the separated components of the mixture.
The separated components of the mixture may be supplied to the surface of the tape or wire from an aperture in, for example, the end of a supply tube the end of which may be cone-shaped. The cone, such as a titanium cone, comprising an oxidised surface coating, may be provided on the opposite side of the tape or wire with the tip of the cone directed towards the aperture. The cone draws off surplus mixture by directing the surplus mixture away from the tip of the cone, down the sides of the cone, to collection means. The oxidised layer on the surface of the cone improves the wettability of the cone and improves the ability of the cone to direct surplus mixture away from the wire or tape, thus preventing diffusion of the separated components along the wire or tape. Titanium is especially preferred as the material for the cone.
The aperture and cone may be positioned so that a continuous stream of separated components joins the aperture to the cone, with the wire or tape passing through the stream.
A strongly dispersive coating may be provided each side of the aperture providing the separated components. This prevents build up, by evaporation, of contaminants, which would ultimately contaminate the separated components, causing random noise. PTFE is especially preferred as the coating. This may be used in combination with the cone, described above.
The conveyor, such as wire or tape, may be subjected to heat prior to entry into the detection apparatus or analyser chamber to vaporise solvent from the component on the conveyor or wire.
Alternatively a nebuliser may be used to spray the sample onto the conveying means. Preferably the nebuliser is a heated nebuliser.
The distance between the nebuliser and the surface of the conveying means may be adjusted so that at least some of any solvent with the sample evaporates before the sample contacts the surface of the conveying means. This allows the amount of diffusion of the sample on the conveying means to be controlled.
Preferably the nebuliser comprises an inner tube through which the sample is passed, an outer tube through which a gas, such as air, is passed, and a nozzle where the gas and sample are combined and through which they are propelled. The outer tube may comprise heating means for preheating the gas and sample stream.
Means for making the mixing of the gas and sample turbulent, thus improving the mixing, may be provided. Such means include vanes for causing the gas to rotate prior to, or after, leaving the nozzle. Alternatively, the outside of the inner tube may be shaped, e.g. in the form of a helix, to rotate the gas flow.
In either aspect of the invention or in any of the consistory clauses relating thereto, the component on the conveyor or wire may be subjected to pyrolysis to convert the component to a gas or sol such as smoke, the pyrolysis preferably taking place within or adjacent to the analyser chamber itself. The pyrolysis may be effected by heating coils disposed adjacent the conveyor or wire or by induction heating or by other means such as laser heating.
The analyser chamber may be defined in a housing formed from a machineable glass or ceramic or from PTFE or PEEK (polyether ether ketone), or from a metal such as stainless steel, copper or brass.
Preferably, one or more gas chromatography detectors such as argon ionisation detectors, are used to detect the pyrolysed components given off by heating. Argon ionisation detectors are discussed in detail in the work by Scott R. P. W. et al, Chromatographic Detectors, Chromatographic Science Series, Vol.73, Marcel Dekker Inc. (New York), pages 119-147 (1996).
In noble gases, such as argon, the outer octet of electrons is complete and as a consequence, collisions between argon atoms and electrons resulting from an ionisation source are perfectly elastic. Consequently, if a high potential is set up between two electrodes in argon, and ionisation is initiated by a suitable radioactive source, electrons will be accelerated toward the anode and will not be impeded by collisions with argon. However, if the potential of the anode is great enough, the electrons will have sufficient kinetic energy that on collision with an argon atom energy can be absorbed and a metastable atom can be produced. The metastable argon atom carries no charge but has an electron displaced to an outer orbit. This gives the metastable atom sufficient energy (ca. 11.6 eV) to ionise most organic molecules. Upon collision with an organic molecule the electron in the outer orbit collapses back to its original orbit, followed by expulsion of an electron from the organic molecule. The electrons produced by this process are collected at the anode, resulting in a large increase in anode current which is measured.
Accordingly the analyser chamber is preferably provided with a source of a noble gas, such as argon, an anode, a cathode, a potential between anode and cathode, and a radioactive source. The radioactive source may be an alpha-particle emitter, such as Americium, or a beta-particle emitter, such as Ni63. This form of sensor is very sensitive, and is universal in its response and does not have the sensitivity difficulties associated with using different solvents with other forms of sensor. A change in current flowing between the electrodes indicates the presence of separated components within the analysis chamber. This may be recorded for example graphically or on a computer.
Other inert gases, such as helium, neon or krypton, or a mixture thereof, may be used instead of argon.
If desired, the invention may include providing further means for analysis downstream of the analyser chamber. The method of the invention is particularly advantageous in that respect as the pyrolysed sample is not further destroyed in the analyser chamber and can be passed for further analysis after the initial analysis has been completed. The further analysis may be performed by detectors such as Electron Capture, Phosphorous and Sulphur selective detectors, Atomic Absorption Spectroscopy and Time of Flight mass spectrometry.
The argon ionisation detector may be omitted and one or more of the other detectors described above may be used instead.
The provision of an electron capture detector downstream of a argon gas detector is especially preferred. Such detectors are discussed in the book by Scott R. P. W. (Supra). Such detectors operate on a different principle from that of argon detecotrs. A low energy xcex2-ray source, such as Ni63 is used to produce electrons as ions. Sensors can be made to function in a D.C. mode with a constant current applied across the sensor electrodes, or as pulsed mode, in which a pulsed potential is used.
In the D.C. mode, a constant electrode potential of a few volts is employed that is just sufficient to collect all the electrons that are produced and provide a small standing current. If an electron capturing molecule (for example a molecule containing a halogen atom with seven electrons in its outer shell) enters the sensor, the electrons are captured by the molecule and become charged. The mobility of the captured electrons are much reduced compared with free electrons and, furthermore, are likely to be neutralised by collision with any positive ions that are also generated. As a consquence, the electrode current falls dramatically.
In pulsed mode, a mixture of machine in argon is usually used as the carrier gas. The xe2x80x9coff periodxe2x80x9d of the potential allows electrons to re-establish equilibrium with the gas resulting in improved sensitivity.
Electron-capture detectors are extremely sensitive and are widely used in the analysis of halogenated compounds. Thus the preferred combination of an argon detector with an electron capture detector combines the universality of the argon detector with the specificity of the electron-capture detector in the same apparatus. The electron capture detector may be placed immediately downstream of the argon detector.
The inventors have found that some samples, after having been pyrolysed, appear to be charged, thus reducing the sensitivity of, for example, the argon detector. Accordingly, means to neutralise the charge on the pyrolysed sample may be provided. This may be, for example, provided by means of an electron capture detector, for example, before the argon detector.
The inventors have found that pyrolysis is less suitable for some highly oxygenated compounds, such as sugars, which do not pyrolyse completely, but instead tend to form residues which remain on the conveyor. The high temperature which is required to completely pyrolyse such residues can ionise the pyrolysed compounds, resulting in difficulty in accurately detecting the compounds in, for example, argon detectors.
According to a further aspect of the invention there is provided a method of detecting components of a mixture separated by chromatography, comprising the step of, depositing sequentially on a conveyor the separated components of the mixture, and moving the conveyor and each compound thereon into an analyser chamber in which analysis of each sample takes place; wherein the analysis chabmer is an argon detector comprising a source of noble gas, such as argon, an anode, a cathode, a potential between the anode and cathode, and a radioactive source.
The conveyor, method of supplying the separated components onto the conveyor, and the chamber housng may be as defined previously for the first and second aspects of the invention. Preferably the radioactive source is an alpha-particle emitter, such as Americium, or a beta-particle emitter such as Ni63.
In this preferred embodiment the separated components react with metastable argon in situ. The electrons emitted by the separated components are detected by the increase in current at the anode, as discussed previously.
The argon gas, once it has passed through the chamber, may be directed to an electron capture detector, and/or other detectors as discussed previously, for further analysis.
According to a further aspect of the invention there is provided apparatus for carrying out the methods according to the invention and in accordance with any of the consistory clauses related thereto.
One difficulty experienced by the inventors has been sealing the aparatus. The detectors used are extremely sensitive, hence, if the apparatus is not properly sealed, false readings may be produced from components in the surrounding atmosphere. Conventional seals of rubber, ceramic or metal surrounding the conveyor rapidly wear out, owing to the abrasive nature of the conveyor. The inventors have therefore developed an improved seal.
Accordingly, a still further aspect of the invention provides a seal for an aperture comprising a plurality of magnetisable particles maintained in a sealing position by one or more magnet means. The seal may be for use with a conveyor which moves through it.
The magnetisable particles may be iron, iron coated with latex, or other commercially available magnetisable particles. The magnetisable particles are maintained in a sealing position by, for example, one or more iron or ceramic magnets or electro magnets. The magnetic field from the or each magnet keeps the magnetisable particles in a sealing position such as in the form of a xe2x80x9cslugxe2x80x9d of particles through which th conveyor passes. Preferably the particles are selected so that if any do escape into the analyser chamber(s), they are not detected or do not interfere with the analysis.
Another aspect of the invention provides a seal for an aperture comprising magnetic particles capable of being maintained in a sealing position by one or more magnet means.